The average maximum temperature during December 2015 was 13.3 Celsius (56F), and that was 5.2 Celsius (9F) above normal for the month. The average minimum temperature was 9.7 Celsius (50F), and that was 6.4 Celsius (11.5F) above the monthly normal. There has never, in the history of temperature measurements, been a December in the London area so mild. The previous record, back in 1974, was almost 3 Celsius (5F) cooler than December 2015. The magnitude of the positive anomaly can be fully appreciated by the addition of further facts. The mean temperature in December 2015 exceeded that of every November, January, February and March since 1900 and has only been bettered in 3 Aprils.

The highest temperature recorded in December was 16.5 Celsius (62F) on the 19th. This was the highest December temperature since records began at this site in 1988 and the highest recorded in this part of Greater London since 1985. The previous record of 15.2 Celsius (59F) measured in December 1997, was also broken on the 17th with 15.8 Celsius (60F) logged. In total, there were 10 days when the temperature exceeded 14 Celsius (57F), including 4 consecutive days between the 25th and 28th. There were no cool days during December and there were only 5 days when the temperature failed to exceed 12 Celsius (54F). The second week of the month contained 4 of these less mild days, but remarkably there were no days when the maximum temperature was less than 10 Celsius (50F). The only 2 days when the temperature stayed below 11 Celsius (52F) were the 13th and 14th. On both days the maximum was 10.2 Celsius (50F), a mere 2.1 Celsius (3.8F) above the average for a normal December day!

There were many mild nights during December with a total of 15 nights having minima above 10 Celsius (50F). On 10 of these milder nights the temperature stayed above 12 Celsius (54F) and this included the 25th/26th, the only night when the temperature remained above 13 Celsius (55F). The low of 13.6 Celsius (57F) was once again a new December record high for this site. There were no cold nights during the month; in fact every night had minima above the long-term average. Only 6 nights recorded minima below 7 Celsius (45F), with 5 of these cooler nights occurring during the first half of December. There were just 2 nights when the temperature dipped below 5 Celsius (41F), the 8th/9th, with a low of 4.0 Celsius (39F), and the 11th/12th when 4.6 Celsius (41F) was recorded.

Frost

There were no air frosts during December, and that's 8 below normal. There were 2 grass frosts, and that's 12 below normal. Both grass frosts were slight and occurred on the nights of the 8th/9th and 11th/12th. The minimum grass temperature on both nights was minus 0.9 Celsius (30F).

Rain

A total of 46.3 millimetres (1.8 ins.) of rain fell during December, and this was almost 30% below the long-term average. Despite the lack of rain, there were 22 days with significant* rain and this was 6 above average. There were 14 wet** days during December and that was 3 days above average for the month. There were only 3 completely dry days with just a trace measured on 6 other days. The first 2 days of December each had a trace of rain, and after 2 millimetres (0.1 ins) on the 3rd a rare dry day occurred on the 4th. It then remained unsettled until the 18th although rainfall amounts were mostly small with only the 10th, when 6.6 millimetres (0.25 ins) fell, having over 5 millimetres (0.2 ins) of rain. The 18th and 19th were the remaining 2 dry days and then the rest of the month stayed changeable. However, daily rainfall totals were small, with only the 30th having over 5 millimetres (0.2 ins) of rain. This was the wettest day of the month with a total of 8.3 millimetres (0.3 ins).

*Significant rain is defined as one that has 0.2mm or more. ** A wet day is defined as one that has 1mm, or more, of rain

Thunderstorms

There were no thunderstorms in Morden during December.

Hail

No hail was observed in Morden during December.

Snow

No sleet or snow was observed in Morden during December.

Wind

South or southwesterly winds accounted for 26 out of the 31 daily observations during December with 3 days having south-southeasterly winds and the remaining 2 having a west-southwesterly direction. The extraordinary persistence of the south to southwesterly winds was a governing factor in generating the high temperatures, but there was little let up in the moderate or fresh strength which inhibited radiative cooling under any clear skies. There were only 6 days when gusts failed to exceed 20 knots (23 mph), and on 7 of the breezy days there were gusts of 30 knots (34 mph) or more. The windiest weather was generally towards the end of the month with 5 out of the last 11 days having gusts of 30 knots (34 mph) or more. The breeziest days were the 5th and 21st when gusts of 36 knots (41 mph) were recorded.